An electrophotographic photoreceptor forms an electrostatic latent image upon charging followed by imagewise exposure to light. The electrostatic latent image is visualized by a toner, transferred to a transfer sheet, e.g., paper, and fixed thereon to make a copy.
Various materials have hitherto been proposed for the photoreceptor, including inorganic materials, such as selenium, zinc oxide, and cadmium sulfide; and organic materials. Among the currently employed electrophotographic photoreceptors using organic materials, i.e., organic photoreceptors, those of separate function type which comprise a combination of a material having excellent capability of generating charges and a material having excellent capability of transporting charges became more popular than those composed of a single material. The charge generating material to be used in the organic photoreceptors of separate function type includes bisazo pigments, phthalocyanine pigments, benzopyrylium dyes, perylene pigments, etc., and the charge transporting material to be used includes pyrazolines, pyrazones, polyvinylcarbazoles, etc. Satisfactory performance as photoreceptor cannot be attained merely by combining these charge generating materials and charge transporting materials at random, and a choice of materials should be made considering various electrophotographic characteristics, such as charge injection properties.
A number of proposals for the organic photoreceptors of separate function type have been made. For example, Japanese Patent Application (OPI) No. 105536/74 discloses an organic photoreceptor employing a combination of a squarylium pigment as a charge generating material and a triarylpyrazoline compound as a charge transporting material. (The term "OPI" as used herein means an "unexamined published application".)
However, the conventionally proposed organic photoreceptors have some disadvantages leaving room for improvement. That is, the conventional ones do not fully satisfy the performance requirements demanded for electrophotographic photoreceptors including charging characteristics, such as charge retention, dark decay, and residual potential, and photosensitive characteristics, such as sensitivity. Particularly when compared with selenium type photoreceptors, the organic photoreceptors are inferior in these characteristics, and the application of the organic photoreceptors is limited mainly to slow copying machines.